A number of cellular communications systems are currently being installed to provide communications facilities to mobile terminals. In such systems, mobile terminals are served by base stations via a radio or air link, each base station defining a system cell. In order that a subscriber may be identified as an authorised system user and located so as to receive calls or other system services, the system is provided with a central database containing subscriber information which can be accessed as required. In the standardised European system commonly referred to as the GSM system, this central store is known as the home location register (HLR). The number of subscribers to mobile communications systems is rapidly increasing, and service providers are becoming concerned that their systems should have sufficient capacity to accommodate these subscribers. A particular problem is the need to store information relating to subscribers on the system home location register, as it will be appreciated that the installed storage capacity of a home location register is finite. Although systems have been designed to allow for future expansion, the unexpected growth in the number of mobile subscribers will soon exceed the storage limits of the present home location registers. One solution to this problem is the installation of a larger home location register to replace existing equipment. However, this is generally undesirable both on the grounds of cost and the possible system disruption that can occur during installation. An alternative solution is the provision of second home location register to which new subscribers are allocated when the existing register becomes full thus effectively doubling the storage capacity. It has been found however that this introduces a routing problem as each request then requires the inclusion of a code indicating the identity of the particular home location register to which the subscriber has been allocated. This requires modification of the network routing tables to accommodate this additional information and can lead to a complex routing which is difficult to manage.